SWIMMING BEHAVIOUR OF SCHOOLS RELATED TO FISH CAPTURE AND ACOUSTIC ABUNDANCE ESTIMATION by OLE ARVE -MISUND DR. PH/LOS. THESIS DEPARTMENT OF FISHERIES AND MARINE BIOLOGY UNWERSITY OF BERGEN BERGEN, NORWAY, 1991 3 CONTENTS Page PREFACE 5 1 LIST OF PAPERS 7 2 SYNTHESIS 9 2.1 Introduction 9 2.2 Swimming behaviour: a basic element in the definitions, 9 structure and functions of schools 2.3 Swimming behaviour of schools during vessel influence, purse 11 seining, and pelagic trawling 2.4 Operation and gear optimization 13 2.5 Swimming behaviour and estimation of school size 14 2.6 Summary 15 2.7 References 15 2.8 Erratum 19 3 PAPERS 1- 7 21 5 PREFACE Discussions about the schooling of fishes is known back to Aristoteles about 2400 years ago. Still, observers are astonished by the amazing regularity and synchrony of schooling individuals. During the last decades much effort has been devoted to reveal the behavioural rules of schooling. There has been significant progress in investigating the functions of schooling behaviour and describing and quantifying school structure. Especially the approach of behavioural ecology of the ways in which behaviour is influenced by natural selection in relation to ecological conditions has been fruitful. Most of the work has been conducted in artificial environments in small aquarias and tanks with a small number of fish. However, due to practical limitations and lack of adequate methods for field observations, there is limited knowledge on the behaviour of large, free-swimming schools. Capture of schooling, pelagic fish is the basis of major worldwide fisheries, and the fishing methods developed are so effective that without effective regulations pelagic stocks may be depleted. Still, capture of sensitive, fast-swimming schools is a challenge that encourage development of new methods and technology. Likewise, there is a need for improvement in in the methodology for acoustic abundance estimation of schooling fish that are avoiding vessels and swimming close to surface. Better knowledge about the swimming behaviour of schools is therefore fundamental, both to improve the catching efficiency and acoustic abundance estimation. This thesis is the result of participation in research activities in which the study of how freeswimming schools are behaving was an important aspect. The projects were conducted by the Institute of Fisheries Technology (FTFI), Fishing Gear and Methods Division, Bergen, and the Institute of Marine Research (IMR), Bergen, from 1985 to 1990. The first two years I was a student at the Department of Fisheries Biology, University of Bergen, and from 1987- 1990, I recieved a scholarship from the Norwegian Fisheries Research Council. During the whole period, I have enjoyed the stimulating atmosphere at the Fishing Gear and Methods Djvision that are now a part of IMR. Motivated by a curiosity on the behaviour of schooling fish obtained as a crew member on board the purse seiner M/S "Nybo" for several periods in the years 1973 - 1980, it has been my intention that the work presented in this thesis may contribute to increased knowlegde of the behaviour of the large, free-swimming schools. 6 Several persons have had major influence on the fullfillment of this thesis. I am particularly grateful to Prof. Steinar Olsen for giving me the opportunity to study and work at FfFI, supporting my work through professional guidance and critisism, and encourage participation in stimulating symposias and meetings abroad. Through his lectures and challenging discussions, Dr. Anders Fem~ has been an inspiring teacher of behavioural biology, and his careful reviews and critisism of the manuscripts have improved this thesis substantially. I owe special thank to Mr. Arvid K. Beltestad and Dr. Asgeir Aglen for their cooperation during most of this work, and my other co-authors are thanked for stimulating discussions. I have appreciated the social atmosphere and go-ahead spirit at the Fishing Gear and Methods Division, and the staff are thanked for help and encouragement during the long process of finishing this thesis. Two of the papers are based on studies conducted on board the purse seiners M/S "Libas" and M/S "Klaring", and the skippers and crews of these vessels are thanked for their good cooperation. I express special acknowledgement to Elen Hals who kindly prepared the thesis and Anders Brettingen who corrected the English text. Bergen, June 1991 Ole Arve Misund To Syn~ve, our daugthers Kristine and Marie, and my mother. 7 1 LIST OF PAPERS Titls thesis is based on the following papers: I Misund, 0 . A. 1990. Sonarobservations of schooling herring. School dimensions, swimming behaviour, and avoidance of vessel and purse seine. Rapp. P.-v. Reun. Cons. int. Explor. Mer, (in press) II Misund, 0. A., W. Dickson and A. K. Beltestad. Optimization of purse seines by large meshed sections and low lead weight; theoretical considerations, sinking speed measurements, and fishing trials. Submitted to Fish. Res. III Misund, O.A. Predictable swimming behaviour of schools in purse seine capture situations. Submitted to Fish. Res. IV Misund, O.A. and A. Aglen. Swimming behaviour of fish schools in the North Sea during acoustic surveying and pelagic trawl sampling. Submitted to J. Cons. int. Explor. Mer. V Eng~. A., 0. A. Misund, A. V. Soldal, B. Horvei and A. Solstad. Fish behaviour and vessel noise; Catch data comparisons, noise analysis, and playback experiments. Submitted to Can. J. Fish. Aquat. Sci. VI Misund, O.A., A. Aglen, A. K. Beltestad and J. Dalen. Relationships between the geometric dimensions and biomass of schools. Submitted to J. Cons. int. Explor. Mer. VII Misund, 0. A. Dynamics of moving masses; variability in packing density, shape and size among pelagic schools. Submitted to Anim. Behav. 9 2 SYNTHESIS 2.1. Introduction detennine the characteristics of vessel-generated sound that may elicit avoidance reactions. Schooling fishes, such as most of the clupeoids, Due to acoustic shaddowing, the directivity of scombroids, and carangids are the foundation of the fish backscattering strength and considerable major worldwide fisheries and fishing industries. noise reverberation in horizontal guided beams, Fishing for sensitive and fastswirnrning schools is estimation of school biomass by sonar echo still a special challenge to the fishennen, and integration is complicated (Mitson 1983). To catching techniques based on surrounding or towed establish an alternative based on the principle that nets develop continously (von Brandt 1984). school volume is proportional to the number of Likewise, acoustic abundance estimation of individuals and the cube of their length (Pitcher and schooling species may be rather uncertain (Aglen Partridge 1979), relationships between the geometric 1989), and special methodology must be applied to dimensions and biomass of schools have been compensate for vessel avoidance and distribution studied Special attention has been paid to close to the surface (Hewitt et al. 1976). investigate the variation in internal packing density Computeriz.ed sonar equipment has become an and shape of schools. indispensable tool for school fishing. Its function is not just to detect underwater fish concentrations, but 2.2 Swimming behaviour: A basic also to reveal the swimming behaviour of schools. element in the definitions, structure Exact and favourable positioning of the gear relative and functions of schools to the avoiding schools is practically totally dependent on such instruments. True motion, Schooling behaviour is quite common among fishes. multibeam sonars are also excellent tools for Shaw (1978) has estimated that about 25% of the quantification of size, swimming behaviour, and approximately 20000 fish species are schoolers. vessel avoidance of schools (Bodholdt and Olsen Moreover, about 80% of all fish species exhibit a 1977). 1be most sophisticated instruments project schooling phase in their life cycle (Burgess and the school area and display the movements of both Shaw 1979). Schooling as juveniles is especially vessel and school (Bodholdt 1982). prevalent Other aquatic organisms such as squids Being in motion is a prerequisite for the (Hurley 1978), tadpoles (Wassermg et al. 1981), and existence of schools (Partridge l 982a, Pitcher krill (Strand and Hamner 1990) may also form 1983). Knowledge and understanding of the school schools. In addition, the behaviour of airborne bird movements are therefore necessary for developing flocks during migrations is a close analogy to fish gears and tactics for fish school catching (Pitcher schooling (Major and Dill 1978). 1979). Infonnation about swimming behaviour is Schooling is regarded as an efficient way of also fundamental to acoustic methods for estimating conducting underwater movements, and is beneficial school biomass (Foote 1989). The first objective of to the individual participants. Effects of schooling this thesis was therefore to quantify the movements such as reduced detection probability, attack dilution of schools when influenced by vessels during fishing and a repertoire of cooperative anti-predator tactics and acoustic surveying. Secondly, the fish-to-gear enhance a higher survival towards predation (Godin reactions are mapped when purse seining and 1986, Pitcher 1986). Higher aggregated search rate pelagic trawling. 1be investigations have been results in faster food locali7.ation for fish in larger carried out by the use of true motion sonars onboard schools (Pitcher et al. 1982), and feeding sites are survey and commercial